What size spacer best for heat soak slant 6 Carter

Use the 5/16" thick carb base gasket with grommetted mount stud holes; Walker № G850 or equivalent. Hard to find at retail any more; see if The Carburetor Shop will sell you one.

Do the fuel line mod, which helps a lot with this kind of hot-fuel issue.

Make sure your manifold heat control valve is free—not stuck—and working correctly.